Field of the Invention
The present invention relates to a printed circuit board which interconnects electronic parts such as an IC and a LSI circuit, and a method of manufacturing the same.
Description of the Related Art
There is a problem that failure of a printed circuit board such as disconnection and short circuit occurs due to corrosion or electrolytic corrosion. Specifically, in a factory environment where a facility such as a machine tool is used, greasy fumes and smoke fog (mist) of a cutting fluid, generated in the course of shaping a workpiece (object to be worked), intrude into the inside of a housing of electrical/electronic equipment, and adhere to the surface of a printed circuit board on which electronic parts such as a LSI circuit and an IC are mounted. As a result, failure of the printed circuit board occurs due to corrosion or electrolytic corrosion, which causes a failure of the electrical/electronic equipment. In order to secure stable operation of the facility such as the machine tool in factory environment over long period of time, it is very important to improve corrosion resistance of the printed circuit board which is fundamental parts of the electrical/electronic equipment.
FIG. 1 is a view showing a cross-sectional structure of the printed circuit board. In a method of manufacturing the printed circuit board 1, the metal conductor 3 such as a copper foil is stuck onto the surface of an insulating substrate 2. A solder resist 4 which protects a wiring pattern is formed on a portion of the metal conductor 3 that has been formed as the wiring pattern. It is general that after the solder resist 4 has been formed, the surface of the metal conductor 3 is subjected to coating treatment with solder or other metal than the solder to protect the metal conductor 3 of the printed circuit board 1, and/or ensure solderability with a metal conductor for the mounting of the electronic parts. Three processes are conventionally used as a surface treatment method, which are (1) hot air leveling (HAL) process, (2) electroless plating process and (3) electrolytic plating process.
FIG. 2 is a view showing a cross-sectional structure of the printed circuit board 1 which has been subjected only to the hot air leveling (HAL) process (1). The hot air leveling (HAL) process (1) is a surface treatment method. In the HAL process, a printed circuit board is immersed into a molten solder bath. After that, excess solder is blown from the printed circuit board with high-temperature and high-pressure air, while the printed circuit board is raised from the solder bath.
However, there is a problem in this process that in the case of a material having high viscosity such as solder, a solder coat (solder thin film) 5 to be formed on the surface of the metal conductor 3 by the hot air leveling (HAL) process does not easily adhere to the metal conductor 3, at a portion 6 of a particularly small gap between the solder resist 4 and the metal conductor 3 such as a gap between the end of the solder resist 4 and the metal conductor 3.
FIG. 3 is a view showing a cross-sectional structure of the printed circuit board which has been subjected only to the electroless plating process (2).
The electroless plating process (2) is inexpensive and can also shorten a plating time period compared to the electrolytic plating process, and so that it has recently been attracted. In this process, when a displacement-type metal plating solution is used as a plating solution, the plating solution for forming a plating film 7 on the surface of the metal conductor 3 is easy to intrude into a gap between the end of the solder resist 4 and the metal conductor 3. Because of this, in the vicinity of this portion, the supply of plating metal ions becomes insufficient, only the oxidation/dissolution of the metal conductor (for instance, copper foil) 3 is easy to occur, and the metal conductor 3 becomes such a state that the portion is scooped out (dissolved) (see Japanese Patent Laid-Open No. 2001-144145). This scooped-out portion 8 becomes a factor of remarkably impairing corrosion resistance.
The description of (3) will be omitted.
The solder coat plays an important role in terms of preventing the corrosion or electrolytic corrosion of the metal conductor 3.
However, as has been described above, there is such a problem in the hot air leveling (HAL) process (1) that the solder coat 5 does not adhere to the gap between the end of the solder resist 4 and the metal conductor 3 on the printed circuit board 1. There is also a problem in the electroless plating process (2) that the scooped-out portion 8 is formed in the gap between the end of the solder resist 4 and the metal conductor 3. In the case where the printed circuit board is used in a factory environment in which the greasy fumes and the smoke fog (mist) of the cutting fluid are scattered, if there exists a portion which is not coated with the solder, there is a possibility that the corrosion or electrolytic corrosion caused by the cutting liquid starts from the portion, progresses and expands.